In recent computer systems, the amount of data to be stored is increasing from year to year. Not only is the number of files increased, but the file size is increasing in e.g. moving picture data of the multimedia system. Consequently, there is a need for a larger data capacity and storage of higher speed. In business organizations or data centers, it is being attempted to increase the storage capacities in order to meet this demand. However, not all data stored in files are accessed, such that one-half or more of the entire data stored are not accessed whatsoever for the latest one year period or even longer.
This phenomenon is taken heed of in the hierarchical storage management (HSM) system in which frequently accessed data is saved in an expensive high-speed low-capacity storage and in which scarcely accessed data are saved in an inexpensive low-speed large-capacity storage.
If the amount of data is small, a system manager or supervisor is in a position to migrate data. However, in the up-to-date state of the art, when terabyte order storage is not uncommon, the storage capacity surpasses the level which would allow a supervisor or system manager to judge the degree of data criticality.
In HSM, data to be migrated is automatically selected from the frequency of access events by the client or from the file information to migrate the data from a high-speed storage to a low-speed storage.
Even when the data is migrated to the low-speed storage, the client is able to access the data, in the same way as before migration, even granting that he/she may feel that the accessing speed is slightly slow.
The conventional HSM system may roughly be classified into a host type system in which the HSM software is installed on the client side and a server type system in which the HSM is carried out on the server side.
In the system in which the HSM software is installed on the client side, the HSM in most cases is application-specified. As typical of this system is the E-mail software coping with HSM. In the case of an E-mail, a simple model that data criticality is decreased as the date becomes old, is more likely to hold and hence it may cope with HSM more readily. A mail with old date is automatically migrated to a secondary storage and a short-cut file is substituted for a mail file of the primary storage. In the short-cut file is stored a pathname of the file entity on the secondary storage.
When the client reads this file, the HSM-accommodating mail software reads out the contents of the short-cut file to access the secondary storage. Thus, the client does not have to be conscious that the file is in the secondary storage. Meanwhile, the ‘short-cut file’ is the name used in the Windows (registered trademark) and the corresponding name in the Unix (registered trademark) is ‘symbolic link’.
With the shortcut file or the symbolic link, as used in this type of the software, the fact that the file entity has been migrated to the secondary storage cannot completely be hidden from the client.
If the processing is through the HSM-accommodating software, the size of the short-cut file, migrated to the secondary storage, is converted into the capacity of the file entity. However, insofar as the Explorer (registered trademark) of the Windows (registered trademark) is concerned, the file size appears as if it is a shortcut file with the file size of 1 kB.
If a file is to be deleted, the file entity of the secondary storage is deleted with the processing through the HSM-accommodating software. However, insofar as the Explorer (registered trademark) is concerned, it is only the short-cut file that is deleted, while the file entity of the secondary storage is not deleted.
Meanwhile, as a system of the type in which the HSM is carried out on the server side, there is such a system employing a stub file for implementation on the file system level, without specifying to applications.
The difference between the stub file and the short-cut file is that the short-cut file is recognized by the file system, while the stub file is a routine file on the file system and it is the HSM-accommodating software that recognizes the file system.
The HSM carried out on the server side suffers from an inconvenience that the servers must be able to cope with HMS, such that it is necessary to exchange pre-existing servers for new HSM-accommodating servers.
As for an intermediate device (switch) enabling plural file servers to be accessed from a client as a sole file system, reference is made e.g. to Patent Document 1. As for the hierarchical storage device, there is disclosed in Patent Document 2 such a configuration of the hierarchical storage device, supervising voluminous data with plural recording mediums with differing access speeds (primary and secondary storage mediums), and which enables less frequently used data to be automatically stored in the secondary storage mediums. In Patent Document 3, there is disclosed a configuration in which a CIFS (Common Internet File System) protocol filter processes write SMB (Server Message Block) message data, sent from a CIFS client, with striping, and in which, in reading data, the data are read in order alternately from plural servers for restoration.
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2003-203029A
[Patent Document 2]
JP Patent Kokai Publication No. JP-P2001-222450A
[Patent Document 3]
JP Patent Kokai Publication No. JP-P2003-150435A